1. Field of the Invention
The present invention is directed to an improved friction material such as used for automatic transmission clutch plates and bands and particularly to a novel impregnant which provides improved frictional properties, durability, wear and heat resistance, and permits the elimination of asbestos.
2. Description of the Prior Art
Previous friction linings of porous paper-like oil absorbent materials have been used for automatic transmission clutch plates and bands, oil brakes and steering clutches operating in a fluid environment. These linings generally include some asbestos fibers which have good heat resistance and frictional properties. The materials were also generally impregnated with an outer coating of phenolic resin which bonded the fibers together to form a strong hard lining with good wear resistance. The resin, however, reduces the frictional properties and porosity required for proper operation in a transmission fluid. Some clutch plates require grinding down of the resin coating to provide the necessary friction surfaces. The durability of transmission bands, which must withstand rapid reversals, has also been inadequate.
Examples of such prior art materials and processes are found in U.S. Pat. No. 2,819,987, issued Jan. 14, 1958, wherein one of the inventors is the same as in the instant application, and U.S. Pat. No. 2,855,329, issued Oct. 7, 1958. The latter patent mentions the use of ethyl silicate, which has desirable heat resistance and frictional properties, to impregnate a base material of asbestos. The silica is dispersed throughout the base material and is then further impregnated with a bonding resin. This later resin impregnation, however, reduces the desired porosity and counteracts the frictional properties of the silica. In addition, asbestos has recently been found to be harmful as an environmental pollutant and efforts are being made to eliminate its use where possible.